Scanning probe microscopes scan a sample or the surface thereof with a probe and thereby provide measurement data for producing a representation of the topography of the sample surface. In the following text, scanning probe microscopes will be shortened to SPM. A distinction is made between various SPM types, depending on the type of interaction between the measurement tip of a probe and the sample surface. Frequently, scanning tunneling microscopes (STM) are used in which a voltage is applied between the sample and the measurement tip, which are not in contact with one another, and the resulting tunnel current is measured.
In the case of the scanning force microscope (SFM, or AFM for atomic force microscope), the measurement tip is deflected by way of atomic forces of the sample surface, typically van der Waals forces. The deflection of the measurement tip is proportional to the force acting between measurement tip and the sample surface, and this force is used to determine the surface topography. In the article “Controllable off-plane deflection of cantilevers for multiple scanning proximity probe arrays,” Appl. Phys. A (2008), 92: 525-530, DOI 10.1007/s00339-008-4668-y, the authors Y. Sarov, T. Ivanov and I. W. Rangelow describe the manufacture of a measurement probe with a two-dimensional arrangement of 4×32 measurement tips.
In addition to these common SPM types, there are a multiplicity of further device types which are used for specific fields of application, such as magnetic force microscopes or near-field scanning optical and acoustic microscopes.
Typical SPM types may have difficulty analyzing structures on a sample which have a high aspect ratio, i.e. a high quotient of depth or height of a structure to its smallest lateral extent. For this reason, standard SPMs can image deep trenches and steep flanks to only a limited extent. The limiting effect here is the finite radius of the measurement tip of SPM probes, in particular the cone angle thereof. FIG. 1A schematically shows the problem of an SPM probe during the scanning of a web. The dotted lines in FIG. 1B symbolize the difficulties arising during the measurement of the flanks of a high web having steep flanks.
Various approaches are known already for solving this problem. In the article “Three-dimensional imaging of undercut and sidewall structures by atomic force microscopy” in Ref. Sci. Instr. 82, pp. 023707-1 to 023707-5 (2011), the authors Sang-Joon Cho, Byung-Woon Ahn, Joonhui Kim, Jung-Min Lee, Yueming Hua, Young K. Yoo and Sang-il Park describe the pivoting of the AFM measurement head or of the Z scanner by ±40° from the normal of the sample surface so as to lead the measurement tip of the AFM probe along steep flanks, in particular overhanging webs. FIG. 2 schematically shows the pivoting or tilting of the Z scanner. This image was taken from the application document “High Throughput and Non-Destructive Sidewall Roughness Measurement Using 3-Dimensional Atomic Force Microscopy” from Park Systems Corporation (https://www.parkafm.com/images/applications/semiconductors/note/1_Park_Systems_App_Note_Sidewall_Roughness_2012_03_14.pdf).
The technical layout for manufacturing the precision mechanism for pivoting the entire AFM measurement head is enormous. In addition, the space required by the AFM becomes very large. If the fulcrum of the measurement tip is then not exactly eucentric, which is a frequent occurrence, the tilting of the AFM measurement head also results in a lateral displacement of the measurement tip of the AFM probe, which makes the navigation on the sample significantly more difficult.
In a second approach, a measurement tip in the form of what is known as an elephant foot (flared tip) is used instead of a needle-type measurement tip. FIGS. 3A and 3B schematically show such a measurement tip which is guided perpendicular to a web and across it. This principle was first described by the authors Yves Martin and H. Kumar Wickramashinghe in their article “Method for imaging sidewalls at atomic force microscopy” in Appl. Phys. Lett. 64 (19), 9 May 1994, pp. 2498-2500. In addition to the typical way of controlling the movement of the measurement tip in the z-direction (the sample normal), a second way of controlling it in the x-direction (the fast scan direction) is used so as to guide the elephant foot probe over steep flanks or under overhangs.
Elephant foot probes for AFM measurement heads are very difficult to produce and are therefore very expensive. The construction of a second feedback loop furthermore leads to very complex and thus expensive AFMs. In addition, retrofitting existing AFMs with elephant foot probes is possible only with difficulty.
In the article “Analysis and design of thermal double-cantilever bimorph actuators for rotating-type mirrors,” the authors Dong Hyun Kim, Kyung su Oh, Seungho Park, Ohmyoung Kwon, Young Ki Choi and Joon Sik Lee describe in Proc. of MNHT2008, Micro/Nanoscale Heat Transfer Internat. Conf., Tainan, Taiwan, Jan. 6-9, 2008, pp. 1063-1067, a micro-electromechanical system (MEMS) consisting of two bimetal beams for adjusting a micromirror.
The Japanese patent application JP 08-094651 discloses an AFM probe, the cantilever of which has three beams of piezoresistive material. A voltage is applied to the central beam, the deflection of the cantilever is determined from the sum signal of the two external beams, and the torsion or bending of the cantilever is ascertained from the difference signal of the two external beams.
Patent document U.S. Pat. No. 8,458,810 B2 discloses a cantilever of an AFM probe having two materials in an asymmetric arrangement with respect to the longitudinal axis of the AFM probe. The two materials have different coefficients of linear expansion. The measurement tip of the cantilever is hereby deflected in a lateral direction in the case of a temperature change of the cantilever. Owing to the asymmetric arrangement of bimorph materials on the cantilever, a thermal signal portion (lateral deflection) can be isolated from a topographic signal portion (deflection of the cantilever in the normal direction).
The two documents mentioned last deal with the objective of separating thermal from topographical portions in measurement signals supplied by AFM probes so as to hereby allow thermal measurements. The problem when determining steep high flanks or overhanging structures is not addressed by any of said documents.